This invention relates to an iron core material, more particularly to an iron core material which is excellent in the frequency characteristic of magnetic permeability and is high in a magnetic flux density (or magnetic induction).
In the prior art, in electrical instruments such as an electric power converting device, including a device for converting an alternate current to a direct current, a device for converting an alternate current having a certain frequency to another alternate current having a different frequency and a device for converting a direct current to an alternate current such as so called chopper, or a non-contact breaker, etc., there have been employed, as electrical circuit constituent elements thereof, semiconductor switching elements, typically thyristor and transistor, and reactors for relaxation of turn-on stress, commutation reactors, reactors for energy heat accumulation or transformers for matching connected to these elements.
As an example of such electric power converting devices, FIG. 1 shows an electrical circuit of a device for converting a direct current to an alternate current. The electric power converting device as shown in FIG. 1 is constituted of a semiconductor switching element 1, a reactor for relaxation of turn-on stress 2 and a transformer for matching 3. Also shown in FIG. 1 is an alternate current load 4 and a direct current load 5.
Through these reactors or transformers, a current containing a high frequency component reaching 100 KHz or higher, even to the extent over 500 KHz in some cases, may sometimes pass on switching of the semiconductors.
As the iron core constituting such a reactor or a transformer, there have been employed in the prior art such materials as shown below. That is, there may be mentioned:
(a) a laminated iron core prepared by laminating thin electromagnetic steel plates or permalloy plates having applied interlayer insulations;
(b) a so-called dust core prepared by caking carbonyl iron minute powders or permalloy minute powders with the use of, for example, a resin such as a phenolic resin; or
(c) a so-called ferrite core prepared by sintering an oxide type magnetic material.
Among these, a laminated iron core, while it exhibits excellent electric characteristics at a commercial frequency band, is marked by iron loss of the iron core at higher frequency band, particularly increased eddy-current loss in proportion to the second power of a frequency. It has also the property that the magnetizing power can resist change at inner portions farther from the surface of plate materials constituting the iron core because of the skin effect of the iron core material. Accordingly, a laminated iron core can be used only at a magnetic flux density far lower than the saturated magnetic flux density inherently possessed by the iron core material itself, and there is also involved the problem of a very great eddy-current loss. Further, a laminated iron core has a problem of extremely lower effective magnetic permeability relative to higher frequency, as compared with that relative to commercial frequency. When a laminated iron core having these problems is to be used in a reactor, a transformer, etc. connected to a semiconductor switching element through which a current having a high frequency component passes, the iron core itself must be made to have great dimensions to compensate for effective magnetic permeability and magnetic flux density, whereby, also because of lower effective magnetic permeability, there is also involved the problem of increased copper loss.
On the other hand, there is employed as the iron core material a compressed powdery magnetic body called as dust core, as described in detail in, for example, Japanese Pat. No. 112235. However, such dust cores generally have considerably lower values of magnetic flux and magnetic permeability. Among them, even a dust core using carbonyl iron powders having a relatively higher magnetic flux density has a magnetic flux of only about 0.1 T and a magnetic permeability of only about 1.25.times.10.sup.-5 H/m at a magnetizing force of 8000 A/m. Accordingly, in a reactor or a transformer using a dust core as the iron core material, the iron core must be inevitably made to have great dimensions, whereby there is involved the problem of increased copper loss in a reactor or a transformer.
Alternatively, a ferrite core employed in a small scale electrical instrument has a high specific resistivity value and a relatively excellent high frequency characteristic. However, a ferrite core has a magnetic flux density as low as about 0.4 T at a magnetizing force of 8000 A/m, and the values of magnetic permeability and the magnetic flux density at the same magnetizing force are respectively varied by some ten percents at -40.degree. to 120.degree. C., which is the temperature range useful for the iron core. For this reason, when a ferrite core is to be used as an iron core material for a reactor or a transformer connected to a semiconductor switching element, the iron core must be enlarged because of the small magnetic flux density. But a ferrite core, which is a sintered product, can be prepared with a great size only with difficulty and thus is not suitable as the iron core material. Also, a ferrite core involves the problems of great copper loss caused by its low magnetic flux density, of its great characteristic change when applied for a reactor or a transformer due to the great influence by temperatures on magnetic permeability and magnetic flux density, and further of increased noise generated from the iron core due to the greater magnetic distortion, as compared with a magnetic copper plate, etc.